Are an insidious Tyranid sub-race that infiltrates and subverts human populations. Are large, fast, armoured, four-armed monsters. Aren’t a great challenge for a Deathwatch Kill-team at range but are murder in close combat.
Are a hive-mind race of aliens that consume all biological matter in their path. Are the implacable enemy that will not negotiate or even communicate and can only be held off or delayed. Are a threat to the galaxy potentially as grave as Chaos, if not more so.
Are a race of technologically advanced aliens new on the galactic scene. Believe in a Greater Good that all races, including the Tau themselves, must serve, whether by choice or subjugation. Are an enemy that can potentially be negotiated with.